1. Technical Field
The present invention relates to a power semiconductor device current detector circuit and detection method, and in particular, relates to a current detector circuit and detection method that detect a current flowing through a power semiconductor device such as a diode, thyristor, transistor (bipolar transistor or metal oxide semiconductor field effect transistor (MOSFET)), or insulated gate bipolar transistor (IGBT).
2. Related Art
An IGBT module in which are mounted an IGBT, which is one kind of power semiconductor device, and a free wheeling diode (hereafter called an FWD) is applied to a power conversion device such as an inverter or direct current (DC) chopper circuit.
Generally, an output current detection is necessary in order to control these power conversion circuits. Normally, two methods are widely employed in the output current detection, which are
1: a method using a current detector such as a current transformer or direct current current transformer (DCCT), and
2: a method using a current detecting resistor (called a shunt resistor).
FIG. 9 is a diagram showing an example of a configuration of a heretofore known three-phase inverter device in which a DCCT is used. The DCCT (also called a direct current current transformer) utilizes a ring-like core and a Hall element, which is a magnetic sensor, as shown in JP-A-2000-134955 (FIGS. 1 and 18 of JP-A-2000-134955) shown below. Wiring is passed through the core, and the current is detected by detecting magnetism generated when current is passed through the wiring. In an inverter 101 shown in FIG. 9, the DCCT 105 is installed on the output wiring, and the detected current value is input into a control circuit 102.
FIG. 10 is a diagram showing an example of a configuration of a heretofore known three-phase inverter device in which a shunt resistor is used. A shunt resistor 106 is connected in the lower arm of the inverter 101 shown in FIG. 10. As an output current flows through the lower arm at a timing at which an IGBT 201 of the lower arm comes on, the output current is detected by detecting a voltage drop of the shunt resistor 106 at this time in the control circuit 102. A current detector circuit in which the shunt resistor is used is shown in, for example, JP-A-2003-274667 (FIGS. 1, 4, and 7 of JP-A-2003-274667) shown below.
FIG. 11 is a diagram showing an example of a configuration of a heretofore known chopper circuit in which a shunt resistor is used. The shunt resistor 106 is connected to a ground (GND) line, as shown in FIG. 11, and the voltage drop of the resistor 106 is detected. By detecting the current in this place, the control circuit 102 can detect a current when there is a boost chopper action from a direct current power source 103 in the direction of an aluminum electrolytic capacitor 108, and a current when there is a step down chopper action from the aluminum electrolytic capacitor 108 in the direction of the direct current power source 103.
The heretofore known current detection methods have the kinds of problem indicated below. That is, with the current detection method in which a DCCT is used shown in FIG. 9, there are problems in that the price is generally high because the DCCT uses a Hall sensor and a core, and that the output characteristics of the DCCT change due to the ambient temperature. Also, there is a problem in that the size of the detector increases because a core is used, and this restricts the miniaturization of the power conversion device.
Also, with the current detection method in which a shunt resistor is used shown in FIG. 10, there is a problem in that, as power loss occurs due to the shunt resistor, the power conversion efficiency of the power conversion device decreases. Furthermore, there is a problem in that the size of the resistor itself increases in order to tolerate a large loss, and this restricts the miniaturization of the conversion device.
However, there are cases in which a current detecting function called a “sensing function-equipped power semiconductor device” is installed in a diode, thyristor, transistor, or the like, which are power semiconductor devices. For example, among IGBTs or FWDs configuring an inverter device, there are ones called “sense IGBTs” and “sense FWDs” equipped with a current detecting function. Hereafter, the function will be described with a “sense IGBT” as an example.
Normally, an IGBT is configured of a few thousand to a few tens of thousands of cells of the same structure. One portion of these is utilized as current detecting cells. In this case, an IGBT formed from the current detecting portion of the cells is called a “sensing region”, and an IGBT formed from the other cells a “main region”. Then, generally, it is often the case that a ratio Nm/Ns between a main region cell quantity Nm (Nm is an integer) and a sensing region cell quantity Ns (Ns is an integer) is set at a few thousand to one. Although the main region and sensing region share a collector terminal, emitter terminals are divided into a main emitter terminal (hereafter called a main terminal) and a current detecting emitter terminal (hereafter called a sense terminal). An IGBT with this kind of configuration is represented by the kind of symbolic circuit shown in FIG. 13A. Also, an equivalent circuit thereof is represented as in FIG. 13B.
In the case of a free wheeling diode (FWD) too, one portion of the chip is divided off for current detecting in the same way, and anode terminals are divided into a main anode terminal (hereafter, a main terminal) and a current detecting anode terminal (hereafter, a sense terminal).
With an existing intelligent power module (IPM), an example has been reported wherein the function of the sense IGBT is used for detecting an overcurrent. That is, in JP-A-10-32476 (FIGS. 8 and 9 of JP-A-10-32476) shown below, it is determined that an overcurrent is flowing through the main region when the current flowing through the sensing region exceeds a predetermined value.
Technologies of detecting current utilizing the sensing region are also proposed in JP-A-2000-134955 (FIGS. 1 and 18 of JP-A-2000-134955) and JP-A-2003-274667 (FIGS. 1, 4, and 7 of JP-A-2003-274667) shown below. These technologies will be described using FIG. 12.
In principle, a current in accordance with the ratio Nm/Ns between the cell quantities flows through the sense terminal (refer to Equation 1).Im/Is=(Nm+Ns)/Ns≅Nm/Ns  (Equation 1)
Herein, Im is the main current (the current flowing through the main IGBT), Is is the sense current (the current flowing through the sense IGBT), Nm is the main region cell quantity, and Ns is the sensing region cell quantity.
Therefore, a shunt resistor Rs is connected to the sense terminal as in FIG. 12, a sense current Is is detected, and the main current is calculated based on Equation 2 below.
                                                        Im              =                                                (                                      Nm                    /                    Ns                                    )                                ×                Is                                                                                        =                                                (                                      Nm                    /                    Ns                                    )                                ×                                  (                                      Vs                    /                    Rs                                    )                                                                                        (                  Equation          ⁢                                          ⁢          2                )            
Although there has been a problem in that resistor loss increases when detecting the main current Im with the shunt resistor Rs, with this method, the shunt resistor loss is small, and consequently, the problems of a decrease inefficiency and an increase in size of the resistor are eliminated.
However, with this method, current detecting accuracy is a problem. That is, in general, a current detecting accuracy of one to two percent is required in an inverter control. However, as the accuracy is low, the method using the sensing region cannot be put to practical use.
Two reasons for the accuracy being low will now be discussed. The first reason involves a factor caused by differences in characteristics between the main region and sensing region. That the current ratio is proportional to the cell quantity ratio is based on the premise that the characteristics of each cell of the main region and sensing region are the same. However, as there is actually variation in the characteristics, the ratio between the main current and sense current is not constant. When illustrating this on a graph, the relationship between the main current and sense current is not linear (refer to the characteristics before correction in FIG. 14).
It can be thought that the difference in characteristics is approximately the difference between threshold voltages Vthm0 and Vths0, and between internal resistances Rm0 and Rs0, of each IGBT.
The second reason for the accuracy being low involves a factor caused by the effect of the shunt resistor. A voltage drop occurs when a current flows through the shunt resistor Rs connected to the sense terminal. For this reason, a difference occurs between the main terminal potential and sense terminal potential, and the ratio between the main current and sense current is not constant.